Wireless sensor systems are becoming increasingly important for multiple applications ranging from security to predictive and condition-based maintenance in a wide variety of applications. Vehicles form one class of systems of particular interest for such monitoring, as they of necessity include many components which are subject to motion, wear, and stress during operation.
Such wireless sensor systems require a source of power. Providing a wired power source significantly reduces the utility of most wireless monitoring applications, while batteries will eventually become exhausted and need to be replaced; this represents significant time and effort and may be very difficult to impossible in some wireless sensor settings (for instance, if a wireless sensor is to be embedded in a component to provide lifetime monitoring). Power harvesting—deriving energy for the sensor from some aspect of the sensor's environment—has thus been of considerable interest to the industry.
Multiple sources of energy may be harvested; in open areas exposed to the sun, photovoltaic (solar) energy harvesting is of considerable use. Some applications can harvest heat energy from associated systems, or use wind energy. However, for vehicles and other operating mechanical systems, vibration and motion energy represent the most likely and commonly considered source.
A considerable body of prior art is devoted to devising various means for harvesting this energy. In U.S. Pat. No. 7,009,310 B2, Cheung et al describe an autonomous power source consisting of a coil with a magnet in a low-friction ferrofluidic bearing which permits the magnet to move along a straight or curved tube with respect to the coil, thus generating electrical energy. In U.S. Pat. No. 7,652,386 B2, Donelan et al describe methods for harvesting energy from the movement of joints, based on the relative motion of the components of a biological system (e.g., the motion of the knee joint). In U.S. Pat. No. 7,667,375 B2, Berkcan et al describe a harvesting system comprised of cantilevers of varying resonant lengths which generate energy through vibration by piezoelectric transduction. Rastegar, in U.S. Pat. No. 7,821,183 B2, describes a system which uses motion of some mass to in turn excite cantilevered piezoelectric beams which generate electrical energy. Gao et al, in U.S. Pat. No. 7,936,109 B1, describe another method in which a piezoelectric element on a bendable substrate collides with a housing through which it moves, thereby generating energy. In U.S. Pat. No. 7,977,852 B2, Ward et al describe a method and system for using characteristics of the energy source to vary parameters of the harvesting arrangement to achieve a greater efficiency of energy harvesting. In U.S. Pat. No. 7,986,076 B2, Yoon et al describe a piezoelectric harvesting method which uses a low-frequency impact to generate power. Gieras et al, in U.S. Pat. No. 8,030,807 B2 describe an electromagnetic harvesting system in which a magnet on a cantilever may move relative to a fixed coil and thus generate electrical energy. Nair et al, in U.S. Pat. No. 8,212,436 B2, describe a method for power harvesting using magnetomotive forces in magnetic materials to produce power.